Communication networks have developed considerably in particular because of the democratization of the high-speed internet and the appearance of general-public audiovisual equipment having network connectivity.
It is thus more and more often necessary to setup quasi-permanent network connections between individuals who have for example the same field of interest so that they share information of any type (audio, video, photo, text, etc).
Technologies such as the VPN (Virtual Private Network) technology for meeting this expectation are already known. This technology makes it possible for example to put in communication two networks of the LAN type (Local Area Network), or a network of the LAN type with distant equipment, in order to create a network of the virtual LAN type composed of the union of the two original LAN-type networks or the union of the original LAN-type network with the distant equipment.
To achieve this putting in communication, VPN technology uses a particular encapsulation method called “tunneling”, which creates a tunnel between a bridge in an LAN-type network with an element to be connected to this network (for example another network provided with a similar bridge or an item of equipment).
This operation consists of encapsulating a level A protocol (embedded protocol) in a level B protocol (transport protocol) by virtue of an encapsulation protocol C. The transport protocol B treats the protocol A as if it were a question of useful data.
Bridges that implement tunneling protocols described for example by IEEE 802.1D are already known.
Each Ethernet segment is connected to the bridge by means of a port, each port thus enabling the various network interfaces to be connected, whether these interfaces be real (in the case of a local connection) or virtual (in the case of a connection by tunnel to a distant network or item of equipment).
At each new port opening, the bridge must ensure that it does not introduce a “loop” into the Ethernet network resulting from the interconnection of the various Ethernet networks.
This is because, if the topology of the resulting network describes a loop (that is to say there exist two possible different paths for reaching the same item of equipment), the result would cause a significant unnecessary overload of the various items of equipment on the network, or even total collapse of the network. More particularly, in the case of broadcast frames, a frame received on one of the ports would be duplicated and sent on the other ports of the bridge. Thus, in the case where a loop exists, one of the frames sent by the bridge would then return to it on one of its ports and this frame would then in its turn be duplicated and re-sent over all the ports of the bridge (other than the one by means of which the bridge received it). In this situation, the sending of a single frame will congest the network by the sending of these different replicas. This phenomenon, known as broadcast storm, is highly detrimental to the stability of the network.
There is already known, for preventing the creation of a loop, a protocol included in the IEEE 802.1D protocol called a Spanning Tree Protocol. This protocol transforms a network topology of the graph type (having redundant paths) into a tree (with no such paths). This protocol makes provision for closing certain ports on certain bridges of the network in order to cut the redundant paths.
To do this, this STP protocol determines a “root bridge”, and then guarantees a loop-free path from the root bridge to all the other bridges in the Ethernet network. When on a bridge two ports are detected as forming part of a loop, one of the two ports goes into a state preventing frame relay via this port (the port is then blocked).
In order to determine the ports forming part of a loop, the root bridge sends a packet called BPDU (Bridge Packet Data Unit), which is relayed by the network bridges. If, on a bridge, this packet is received at more than one port, this means that there exists a loop between these ports and only one of these ports must remain open.
However, since the relaying of the BPDU packets by the network bridges are not immediate (relay subject to a time delay typically equal to 2 seconds) and the topology of the network may be complex, the STP protocol defines a waiting period (“Forward Delay”). During this time the bridges must await (listening phase) any BPDU packets on their various ports in order to detect the possible presence of loops.
This waiting period has a default value defined by IEEE 802.1D standard and which is 15 seconds.
Improved protocols are already known (such as the RSTP protocol defined by IEEE 802.1W standard or the protocol defined in patent U.S. Pat. No. 6,628,624) in which the bridge is parameterized in advance so that certain ports open without a waiting period when the network topology so permits.